Calvin often pretends he is someone else and often gets lost in imaginative landscapes. His hyperactive imagination leads him to envision himself as other characters with different abilities and goals to escape difficult situations such as school quizzes. The strips which portray these events blend fact and fantasy as Calvin morphs elements in the "real world" with his fantasies. On several occasions, Calvin has appeared as either a larger or a smaller version of himself, wreaking havoc like Godzilla or crawling across a book page as "Calvin, the human insect."[1] More frequently, Calvin's imagination transforms him into a being of a different kind. In many comics which involve Calvin in an alter ego, the strip is heavily stylized to portray Calvin's environment from his imaginative point of view. Such strips often finish with the last panel or two back in the "real world," providing backstory or simply to show the real world consequences of Calvin's actions. (i.e., Spiff dousing an "alien" with a "Hydro Bomb" (e.g. a glass of water), who is revealed to be Calvin's father).

The Calvin and Hobbes Tenth Anniversary Book provides background on Spiff's character. Watterson first conceived an earlier version of Spiff when he was taking a high school German class, producing a two-page, short-lived comic titled "Raumfahrer Rolf". When he was in college he reworked the strip and renamed his hero "Spaceman Mort", but decided not to publish it for various reasons. Later on, after finishing college, Watterson came up with the name "Spaceman Spiff" and made what he hoped was a professional strip with Spiff as a hero. There was little resemblance to the Calvin-Spiff character: The early Spiff was a "diminutive loudmouth" with a Charlie Chaplin moustache who explored space in a dirigible with his sidekick Fargle. All the newspaper syndicates rejected this early strip, and the present Spiff was finally born as one of the many imaginary alter egos of Calvin when the Calvin and Hobbes strip began.[2]:68

Spaceman Spiff, "interplanetary explorer extraordinaire," explores the outermost reaches of the universe "by popular request" in a red flying saucer with a bubble canopy (resembling a UFO). As the comic developed and evolved over time, Watterson began to introduce Spiff's adventures with alliteration, featuring phrases such as, "Poised precariously over a percolating pit of putrid pasta."

The galaxy in which Spiff travels is a cruel place where Spiff is shot down or captured by ferocious and disgusting aliens. In reality, these aliens are often people such as Calvin's parents or Miss Wormwood. Frequently, Spaceman Spiff becomes stranded on an unexplored planet due to alien attacks or malfunctioning equipment. Most of these planets seem devoid of civilization, and often have hostile environments or alien predators. Watterson based his alien landscapes on the rock formations of southern Utah and landscapes within the comic Krazy Kat.[3] Gradually, the monsters became more detailed.

The vocabulary, and Spiff's array of high-tech gadgetry, offered a caricature of the "science" found in many science fiction books and TV series. Watterson described Spaceman Spiff as a parody of Flash Gordon. The grand "space opera" style of Spiff's adventures may also spoof Star Trek and Star Wars. Since all the Spiff adventures have a lone protagonist playing with reality, they are close to the early work of Philip K. Dick and that of other writers who have featured lone individuals going to the edge of their perceived world.

In the final years of Calvin and Hobbes, Watterson began to show an interest in information technology, often pitting the progressive and computer-savvy Calvin against his technophobic and reactionary father. Watterson's satire of the personal computer and its effects spilled over into the Spaceman Spiff strips. In one strip, Spiff's ship had a computerized weapon control system that was so finicky and slow that Spiff was hit by the aliens before he had a chance to use any of his weapons.

In the Introduction to Opus: 25 Years of His Sunday Best, fellow comic strip artist Berkeley Breathed noted that in 1981, he had begun a storyline for his character Milo Bloom in Bloom County, which he described as 'a little blond-haired boy with an over-tweaked imagination working out his real-life anxieties and passions via space hero fantasies'; had it continued, it would have featured 'Dukakis and Donald Trump aliens'. Daunted by the negative reaction from the fans after the first strip based on this story, Breathed decided not to continue with the story, and began a new storyline by introducing a Penguin who watched the 'Mister Rogers' Show - an idea which sat much better with Bloom County readers. Five years later, Calvin rocketed into his adventures as Spaceman Spiff, which Breathed says 'was just how the comic universe was meant to be'.

Spiff carries a futuristic sidearm, originally named the AtomicNapalm Neutralizer. After the first few strips, the name was changed to the simpler Death Ray Blaster, or Death Ray Zorcher. It is difficult to say whether it was a replacement weapon or not, since both guns were similar in shape; on one occasion Spiff also used a similar one called an Atom Blaster. The Death Ray Blaster had various cooking-based settings ("Shake-n'-Bake", "Medium Well", "Deep Fat-fry","Liquefy" and "Frappe"), yet they prove useless against every enemy Spiff faced. The real-life equivalent of these fantasy weapons often turned out to be a dart gun, water pistol, rubber band, spitballs, or snowballs, explaining their ineffectiveness. Spiff's other weapons include Demise-O-Bombs (waterballoons) a Zorcher (his water gun) at least one Hydro Bomb (a jar or glass filled with water) and one Stun Blaster (a rock).On one occasion he even had a jet pack (though this was just him running in real life).

Spiff's saucer is equipped with an astounding array of weapons, detectors and propulsion devices, many of which tend to malfunction. The design of the ship appears to be based on a toy spaceship of Calvin's, which appeared in one strip. On one occasion, it was shown as being equipped with a computer-guided weapon bearing the same name as the afformentioned Death Ray Blaster. The computer, unfortunately, was so finicky and slow that Spiff was shot down before he could use it.

Spaceman Spiff first appeared in the twelfth strip of Calvin and Hobbes on November 29, 1985.[4]:26 Spaceman Spiff had a ten-page watercolor mini-graphic novel devoted to him in the Lazy Sunday Book. Spiff's last appearance was on October 1, 1995.[5]:441

Stupendous Man is a superhero Calvin often becomes. He, like Spaceman Spiff, narrates his own adventures in the third person. Stupendous Man has several nemeses: Mom Lady (Calvin's Mom), Babysitter Girl (Rosalyn), Annoying Girl (Susie Derkins), and the Crab Teacher (Miss Wormwood). Despite his frequent use of various "stupendous powers", Stupendous Man has admittedly only won "moral victories".

In one strip, Stupendous Man had a battlecry, in which every letter that makes up the word stupendous, stood for something—a parody of Captain Marvel's magic word, "Shazam." This had limited success.

Occasionally, Watterson seemed to use Stupendous Man to parody popular superhero comics with his use of superhuman powers for useless plans, like rotating the Earth around to give Calvin another day off school, a reference to the 1978 film, Superman. In one strip, Stupendous Man is identified as "Six-year-old millionaire-playboy Calvin", which could be a homage to Batman's alter ego Bruce Wayne. Although in most, he is described as "Mild-mannered Calvin", a reference to Clark Kent.

Stupendous Man's powers include high speed flight, "the strength of a million mortal men,"[2]:88 enhanced vision, "ultra-sonic hearing," and superhuman intelligence. Calvin often gives Stupendous Man titles such as "the masked man of mega might."

Watterson first used Tracer Bullet in a story where Calvin has Hobbes cut his hair because, "the barber never cuts it the way [he likes]," and ends up bald. This story turned out to be one of Watterson's favorites: the sight of Calvin's haircut was one of the few times his own work made him laugh aloud, and Calvin's use of a fedora to cover his head led to the introduction of Tracer Bullet. Watterson would later lament, "Would that I could write like this more often" (Tenth Anniversary Book).

Tracer Bullet only appeared twice more. In his second appearance, Calvin has to solve a problem on a math test and imagines the process as Tracer Bullet. He last appeared in a story wherein Calvin and Hobbes break a lamp during a football game.[2]:132–134

Calvin loves dinosaurs and imagines himself as a dinosaur in many of the strips, usually a predator such as a Tyrannosaurus rex[2] on the hunt. Once, he claimed to have discovered Calvinosaurus, a monstrous carnosaur that could devour an Ultrasaurus in one bite. On one occasion, he delivered a "tasteless and entirely uninformative" report on overpopulation, in which Susie Derkins is devoured in the schoolyard by a pack of Deinonychus dinosaurs as a consequence of natural selection."[2] In another strip, he imagines himself as a T. rex terrorizing the art museum his parents dragged him into.[7] He also pretended to be an Allosaurus fighting a straggler in an Apatosaurus herd. His adventures pretending to behave like dinosaurs usually end with him getting in trouble for making loud dinosaur noises in inappropriate places.[2] Watterson remarked that dinosaurs became "one of [his] favorite additions to the strip".[2]

Calvin appears in several strips as an adult, often when he's been roped into playing make-believe with Susie Derkins, and the comic itself is drawn in a much darker and more realistic art style reminiscent of early Judge Parker or Rex Morgan, M.D. strips until Calvin gets fed up with the game, and the real world scene is revealed. According to Calvin and Hobbes Sunday Pages 1985-1995, Watterson expected the reader to wonder for a moment if Calvin and Hobbes had been dropped in favor of another strip. Watterson mentioned that he enjoyed having the adult Calvin (and adult Susie) get into ridiculous dialogue and actions.[2]

Calvin often imagines himself to be a pilot. One time he piloted an F-15 to destroy his school; Watterson noted that the large volume of negative mail to a childish fantasy indicated that "Apparently, some of my readers were never kids themselves."[2] In other strips, Calvin flies in the model F-4 he built (which works so badly that he cannot land or eject),[2] aggressively passing other airliners on the tarmac, taking off, and encountering a collision with a landing airliner (the airliner resembles a McDonnell Douglas DC-10). He is also seen captaining a Boeing 727 competing with a rival airline's pilot for the same runway. In another strip, Calvin imagines himself as a pilot headed for St Louis, Missouri in a McDonnell Douglas MD-80 who then decides he wants to see the Grand Canyon up close—in reality, he is actually in the family car going to the store with his mother. He is also seen running for hours in circles outside the house pretending to be a Boeing 727.